This invention relates generally to a pilot control system and, more particularly, to a pilot control system for maintaining a constant operating hydraulic fluid pressure in a hydraulic system throughout a range of hydraulic fluid flow and temperature conditions.
Machines such as power generation sets, earth working machinery, paving machinery, load transfer carrying machinery, excavators and the like use hydraulic systems to manipulate and control the many components of the machine. These hydraulic systems may be used to manipulate such components as booms, buckets, scrapers, rippers and the like. The hydraulic systems may also be used to maintain an operating pressure in a track tension system of the machine, as well as to provide adequate pressure for steering and braking operations of the machine.
A hydraulic pump, which is regulated by an engine speed of the machine, is used to maintain a desired operating pressure of the hydraulic system and its many components. However, it is well known that the many different components of the machine such as, for example, the boom and the track tension system, require different operating pressures. By way of illustrative example, the track tension system may require an operating pressure of approximately 7000 kpa (1015 psi) while the boom may require a lower operating pressure of approximately 4000 kpa (580 psi). If the operating pressures of these components are not maintained within the proper design parameters, the machine components may be damaged during operations of the machine. It is thus very important to adequately control and regulate the operating pressures of the many components on the machine.
A pilot control system is used to control, regulate and maintain the operating pressures of the many components of the machine. That is, the pilot control system is used to maintain the operating pressure of both a component requiring a high operating pressure (e.g., track tension system) and a component requiring a lower operating pressure (e.g., boom or other component). This is accomplished by providing several relief valves in series, each having different operating parameters. For example, a hydraulic system used to maintain an operating pressure of approximately 7000 kpa (1015 psi) for the track tension system and an operating pressure of approximately 4000 kpa (580 psi) to control the main hydraulic system for the boom, may require a pilot control system having a 7000 kpa (1015 psi) pressure relief valve in conjunction with a set of two lower pressure relief valves (e.g., a 3000 kpa (435 psi) pressure relief valve and a 4000 kpa (580 psi) relief valve downstream of the 3000 kpa (435 psi) relief valve). This arrangement of relief valves provides two operating pressures, one at 7000 kpa (1015 psi) and another at 4000 kpa (580 psi).
However, as is well known in the art of hydraulic systems, the operating pressures of the hydraulic system are dependent on the flow rate and temperature of the hydraulic fluid. Thus, at a lower flow rate or a higher temperature, the operating pressure of the hydraulic system will decrease proportionally. This fluctuation in the operating pressure of the hydraulic fluid upstream of the relief valves affects the operations of the relief valves of the pilot control system; that is, the relief valves are very sensitive to fluctuating operating pressures and thus cannot adequately maintain and regulate the minimum operating pressures (downstream) for many of the components of the machine during these states of operating pressure fluctuations. This inability to maintain and regulate a proper operating pressure for the many components results in damage to the components of the machine.
U.S. Pat. No. 4,126,993 to Grattapaglia et al., which was issued on Nov. 28, 1978, shows a hydraulic system for an earth moving machine and a temperature-controlled valve for a hydraulic system. The hydraulic system includes two relief valves and a temperature control system for monitoring the temperature of the hydraulic fluid. However, the Grattapaglia et al. system does not compensate for fluctuations in operating pressures due to different flow conditions. Also, the Grattapaglia et al. system includes relief valves in line with components of the earth moving machine which are sensitive and unable to maintain adequate operating pressures when the pressure of the hydraulic fluid fluctuates upstream from the relief valves.
The present invention is directed to overcoming one or more of the problems as set forth above.
In one aspect of the present invention a pilot control system has a pilot pump and a pilot relief valve in fluid communication with the pilot pump. A first pressure reducing valve is in fluid communication with the pilot pump and the pilot relief valve. The pressure reducing valve reduces the predetermined supply pressure to a first reduced pressure.
In another aspect of the present invention a machine has a pilot control system. The machine has a track tension system, at least one hydraulic actuator and a pilot pump and a main hydraulic system. The pilot pump supplies hydraulic fluid to the track tensioning system and the main hydraulic system supplies main hydraulic fluid to the at least one hydraulic actuator. A pilot relief valve regulates a supply pressure of the hydraulic fluid and a first pressure reducing valve, in fluid communication with the pilot pump, reduces the supply pressure to a first reduced pressure.